Chromium or “chrome” catalysts, also called “Phillips catalysts,” are oxidized chromium compounds fixed on porous oxide solids that are valuable mainstays for polymerizing ethylene or copolymerizing ethylene with other α-olefins. The catalysts are usually made by impregnating the porous oxide with a chromium (III)-containing solution, evaporating the solvent, and heating the supported chromium compound to 400 to 1000° C. under oxidizing conditions to activate the catalyst and convert the Cr(III) species to a Cr(VI) species (see, e.g., U.S. Pat. No. 2,825,721).
Even after oxidation to a Cr(VI) species, however, chromium catalysts are often not immediately active when combined with ethylene or other monomers. Usually, an “induction” period or time lasting from a few minutes to an hour or so precedes polymerization (see U.S. Pat. No. 7,407,591).
Chromium catalysts are often modified with compounds that contain boron, aluminum, magnesium, titanium, vanadium, fluorine, or other elements. Some of these modified catalysts activate more quickly and have reduced induction times compared with earlier catalysts. For example, the boron-modified chromium on silica catalysts described in U.S. Pat. Nos. 3,780,011 and 4,173,548 provide improved reactivity while maintaining a narrow polymer molecular weight distribution. U.S. Pat. No. 4,374,234 teaches the use of chromium catalysts modified by addition of trialkylaluminum or dialkylmagnesium compounds to reduce the induction time.
Numerous techniques are recommended for activating chromium catalysts, some of which help to reduce the induction time. U.S. Pat. No. 3,281,405, for instance, describes a two-stage air activation process in which the supported chromium catalyst is heated first at 1500 to 2000° F., and then at 800 to 1500° F. in the presence of air to improve productivity while maintaining a desirable melt index. U.S. Pat. No. 4,173,548 teaches to form and activate a boron-modified chromium on silica catalyst at elevated temperature (about 600° C.) in a fluidized bed with a substantially dry reducing gas (e.g., a mixture of nitrogen and carbon monoxide) followed by an oxidizing gas (e.g., air) also at elevated temperature, followed by cooling in an inert atmosphere. The catalyst has improved reactivity. In another approach, taught in U.S. Pat. No. 5,093,300, a fluidized bed of chromium on silica catalyst is activated successively with (1) dry air at 815° C.; (2) dry air at 650° C.; (3) dry nitrogen at 650° C.; and (4) dry nitrogen at 650 to 350° C., followed by cooling to room temperature.
U.S. Pat. No. 6,147,171 teaches to shorten the induction time of a Phillips catalyst by reducing it with an internal alkene, an alkyne, 1,3-butadiene, or an aldehyde. Suitable alkenes include, e.g., E- or Z-2-butene, E- or Z-2-pentene, E- or Z-2-hexene, and similar compounds. Ethylene and α-olefins (“C3-C10 1-alkenes”) are expressly taught as unsuitable reducing agents.
The slurry-loop process for making polyethylene is well known and has long been commercial (see, e.g., U.S. Pat. Nos. 3,248,179 and 3,644,323).
It is well known, of course, to polymerize ethylene in the presence of an α-olefin comonomer, usually to generate polyolefins having reduced densities and substantial amounts of incorporated comonomer units. For example, U.S. Pat. No. 6,465,586 teaches to use 0.1 to 10 wt. % of the comonomer. U.S. Pat. No. 4,374,234 teaches to use 0.2 to 20 wt. % of the comonomer. U.S. Pat. No. 6,632,896 teaches to use 1-13 moles of comonomer per kmole of ethylene, i.e., >0.1 wt. %. Thus, at least about 0.1 wt. % of a comonomer is typically taught for use, and much higher levels are normally used to reduce polyolefin density.
Of particular interest are high-density polyethylene resins commonly used for blow molding milk, water, or juice bottles. These resins require high density (usually at least 0.957 g/cm3) for good stiffness properties (see U.S. Pat. No. 5,198,400). Additionally, the resin must have good organoleptic properties (i.e., taste, odor) so the amount of unincorporated residual monomer present in the resin must be very low or nonexistent. Ideally, such resins could be made more efficiently while maintaining high stiffness and good organoleptics.
In sum, the industry would benefit from improved ways to improve process productivity and reduce or eliminate the induction time normally associated with using oxide-supported chromium catalysts. A valuable process could be used in conjunction with well-known activating techniques. Preferably, the process could be practiced in conventional slurry-loop ethylene polymerizations using existing equipment, well-established operating procedures, and common chromium catalysts and activating reagents. An ideal process could efficiently provide high-density polyethylene resins useful for making blow-molded bottles.